In hydraulic drilling operations, a highly pressurized drilling fluid is discharged through a drill head as a high velocity cutting jet which cuts away the material at which it is directed to form a borehole. As the material is removed, the drill head is advanced to extend the borehole into the earth. The drill head is typically attached to a tubular work string to which the pressurized fluid is applied, and the force exerted on the work string and head by the fluid drives them in the forward direction. The rate at which the drill head advances is limited by a cable which is attached to the work string and played out at a controlled rate.
The use of the restraining cable has certain limitations and disadvantages. It requires not only the cable itself but also a drum or storage reel for the cable and a brake or some other means for controlling the rate at which the cable is played out. Under significant tension the cable (whether holding back the drill string or while being used to retract the drill string) will stretch, making it difficult to precisely control the rate of penetration of the drill string. There is also a possibility that the cable may break, which would necessitate shutting down the drilling operation to recover the drill head, repair the cable, and possibly also repair or replace the drill head in the event that it is damaged by impacting with the formation when the cable breaks.
The method described in U.S. Pat. No. 5,255,750 resolved at least some of the aforementioned concerns through the creation of a “hydraulic brake” which utilized a set of dual acting seals, one sealing the outer diameter (OD) of the drilling apparatus and the second sealing the drilling apparatus inside a continuous cylinder the length of the drilling apparatus. In between these seals, a fixed sized orifice is installed to allow the fluid that is trapped within these seals to escape, thus decreasing this volume of fluid and allowing the drilling apparatus to move at a controlled rate of penetration.
The use of this seals/orifice system had several limitations and disadvantages. The seals have to work in a harsh particle filled environment that is typical of an oil or gas well drilling operation and in some cases leakage of the seals will cause the system to fail. The orifice also has to maintain a very precise size and shape, while operating at high pressures and temperatures and allow abrasive material to pass through without eroding. The combination of length of the drilling apparatus and the continuous cylinder has to be planned before the operation and cannot be altered or the seals will not seat simultaneously. In addition, once the orifice is selected to give a desired rate of penetration, this rate cannot be changed to allow for a faster or slower rate of penetration without bringing the entire drilling apparatus to surface and physically changing that orifice.
In addition, with the seals/orifice method a separate piece of equipment must be used to lower the drilling apparatus from surface into position for the seals to seat. This requires a cable and a drum with the ability to control the rate at which the cable and drilling apparatus is spooled out. Once the drilling apparatus has completed its hydraulic drilling operation, a method must be implemented to retract that drilling apparatus back into the original well (as a minimum) or all the way back to surface to repair the seals or change the orifice.